Modern optical wide-area networks which are resistant to obsolescence must meet high demands with respect to capacity, flexibility, reliability and transparency. These demands on a transport network are met to an optimum degree when the data is transmitted and switched in optical frequency-division multiplex. In the case of optical frequency-division multiplex (wavelength-division multiplex--WDM), a plurality of transmission channels, which are separated from one another by their optical carrier frequencies (wavelengths) differing by several hundred GHz from one another, are concentrated in one fiber. The maximum number of channels possible is limited by the gain bandwidth of optical amplifiers.
Photonic networks having high-bit-rate fiber-based transmission links, optical frequency-division multiplex (wavelength-division multiplex--WDM) and WDM switching networks for semipermanent switching of optical channels as the future transport networks of the telecommunication service providers will initially have to be installed as overlay networks forming the backbone for the networks existing today; The continuously increasing demand for transmission capacity will lead to a continuous expansion of the photonic networks.
The WDM switching networks provided for semi-permanent permanent and non-blocking switching of optical channels are so-called optical cross-connects (OCC). As a rule, such optical cross-connects have both a frequency switching stage and a space switching stage. In this connection, a WDM switching network comprising input WDM demultiplexers, which in each case divide the incoming optical signal from the associated input fiber in accordance with wavelengths, followed by a space switch, adjustable wavelength converters, which convert the respective incoming wavelength to the respective outgoing wavelength, and output WDM multiplexers, which combine the wavelengths supplied to them, is known from ICC'93 Conf. Rec. Vol. 3/3, 1300 . . . 1307, FIG. 10.
From ntz 46 (1993)1, pages 16 . . . 21, FIGS. 13 and 14, WDM switching networks are known in which input WDM demultiplexers (1/N in FIG. 13; unlabeled in FIG. 14), output WDM multiplexers (N/1 in FIG. 13; unlabeled in FIG. 14), wavelength converters (frequency stages in FIG. 13; filters and frequency converters in FIG. 14) for converting a wavelength to the respective outgoing wavelength and an intermediate space switch (space stage in FIG. 13; fiber-optical switch in FIG. 14) are also provided.
From U.S. Pat. No. 5,194,409, a switching network is known in which (according to FIG. 3) wavelength-individual space switching matrices (70) lead to WDM multiplexers (59).
From U.S. Pat. No. 5,194,977, a switching network (23 in FIG. 2) comprising wavelength converters (17-1 . . . 17-4) is known.
The wavelength converters (optical frequency converters) provided in WDM switching networks make it possible to keep down the probability of blocking for connections to be newly set up in photonic networks even with high network usage. However, optical frequency converters represent relatively expensive subsystems, on the one hand, whilst, on the other hand, a connection may not actually require frequency conversion in every WDM switching network. To take account of this, frequency converters which can be inserted into a connection can be provided in a WDM switching network as required.
In the case of a WDM switching network having p WDM demultiplexers, which in each case divide the incoming n-channel WDM signal from the associated input fiber in accordance with wavelengths (optical frequencies), followed by a space switch and p output WDM multiplexers combining the n optical signals supplied to them into an n-channel WDM signal, it is already known in this connection to provide wavelength converters in a wavelength converter pool located between additional outputs and inputs of the space switch and to insert from this pool, only if required, a wavelength converter into a connection conducted via the WDM switching network (OFC'95 Technical Digest, 271 . . . 272; IEEE Communications Magazine, November 95, 84 . . . 88).
Such a WDM switching network is drawn in FIG. 1. Firstly, this WDM switching network exhibits p input ports and p output ports and input and, respectively, output fibers E1 . . . , Ep and, respectively, A1, . . . , Ap connected thereto for WDM signals (optical frequency-division multiplex signals comprising in each case n optical channels and in each case q ports e1, . . . , eq and, respectively, a1, . . . , aq at the input and, respectively, output end for signal-channel signals. The p input fibers E1, . . . , Ep in each case lead to a WDM demultiplexer D1, . . . , Dp which divides the incoming WDM signal (optical frequency-division multiplex signal) at the respective input port in accordance with wavelengths .lambda.1, . . . , .lambda.n (optical frequencies). The maximum total of p.multidot.n outputs of the p WDM demultiplexers are connected to p.multidot.n inputs of an (integrated) optical space switch network R. At the output end, p.multidot.n outputs of the space switching network R are connected to the in each case n inputs of p WDM multiplexers M1, . . . , Mp which in each case combine the n signals of different wavelengths (.lambda.1, . . . , .lambda.n) supplied to them into a WDM (optical frequency division multiplex) signal and which, in turn, lead to the output ports of the WDM switching network and the output fibers A1, . . . , Ap connected thereto.
In a wavelength converter pool KP located between additional m.multidot.n outputs and m.multidot.n inputs of the space switching network R, m.multidot.n wavelength converters K11, . . . , Knm are provided, namely m frequency converters K11, . . . ; . . . , Knm for each of the n wavelengths, where m.ltoreq.p. In the WDM switching network drawn in FIG. 1, the input ports are located on the left in the figure and the output ports of the WDM switching network are located on the right in the case where the input wavelength is variable and the output wavelength is fixed in the wavelength converters, whilst the input ports are on the right and the output ports are on the left in the figure in the case of a fixed input wavelength and variable output wavelength. The drawing of FIG. 1 thus covers both cases.
It is of disadvantage in this WDM switching network that the magnitude (q+p.multidot.n+m.multidot.n).times.(q+p.multidot.n+m.multidot.n) of the space switching network R depends on the number m of the wavelength converters. When the WDM switching network is being installed, its planned maximum extension must therefore be known already; it is not possible to extend beyond this planned value.
Since the blocking probability, and thus the requirement for wavelength converters, only increases with an increasing traffic load, it is desirable to be able to extend WDM switching networks progressively with wavelength converters, and that without impairing the freedom of such an extension due to assumptions and plannings which may already be superseded. It must not be assumed in this connection that a wavelength converter is capable of converting any input wavelength into any output wavelength. Instead, it must be assumed that wavelength converters which can be implemented with economically justifiable expenditure (without continuously tunable filters and/or sources) will convert any input wavelength to a fixed output wavelength (or conversely).